Pulverizer control



July 10, 1962 F. GILBERT 3,043,525

PULVERIZER CONTROL Filed March 10, 1960 INVENTOR.

LYMAN F. GILBERT ATTORNEY United States Patent 3,043,525 PULVERIZER CONTRUL Lyman F. Gilbert, Lyndhurst, Ohio, assignor to Bailey Meter Company, a corporation of Delaware Filed Mar. 10, 1960, Ser. No. 14,009 8 Claims. (Cl. 241-60) This invention relates to control and more particularly to an apparatus and a method for control of the direct firing of pulverized coal wherein one or more air-swept pulverizers each supplies pulverized coal to one or more burners discharging into a combustion zone such as the furnace of a steam generator, the hood of a cement kiln or the like.

In such a system air and coal are injected into the pulverizer. The air injected into the pulverizer is known as primary air and the air-coal ratio, which may be defined as the weight of air per unit weight of coal, is known as the primary air-coal ratio. The same air which sweeps the pulverizer conveys the coal from the pulverizer to the burner and is discharged with the pulverized coal into the combustion zone.

For satisfactory operation and to obtain optimum efficiency it is necessary that the primary air-coal ratio be maintained at some definite value which depends upon the type of pulverizer and burner, the size and configuration of the piping used to transport the air-coal mixture from the pulverizer to the burner and the character of the coal. The correct primary air-coal ratio generally varies with load and usually increases with a decrease in pulverizer output. However, it is well established that a definite and constant primary air-coal ratio should be maintained at any given load.

Ordinarily the coal is dried in the pulverizer, such drying being accomplished by preheating the primary air to the pulverizer. inlet air temperature of upwards of 600 F. may be required to effect proper drying. Preheated primary airis generally obtained from either a gas or a stream air heater which is a component of the unit served by the pulverizer, or where not available in this manner may be obtained from high temperature furnace or flue gas usually tempered with air. For safe operation it is necessary that the coal-air temperature leaving the pulverizer be maintained below a maximum value ordinarily about 160 F. as established by practice. Therefore, to obtain proper drying of the coal with safety the temperature of the air to the pulverizer must be controlled to maintain a predetermined coal-air temperature leaving the pulverizer.

It is a principal object of my invention to provide an apparatus and method for the control of the direct firing of pulverized coal which will effect satisfactory operation and optimum efficiency It is a further object of my invention to provide such an apparatus and method wherein the desired primary aircoal ratio is accurately maintained throughout the range of operation of the system and during load changes.

Still another object of my invention is to provide such an apparatus, and method wherein the optimum coal-air temperature is accurately maintained regardless of rapid and violent changes in load.

A further object of my invention is to provide such an apparatus and method which inherently prevents coal being fed to the pulverizer at a rate in excess of that which With extremely high-moisture coal an ice can be accommodated by the then existing rate of primary air flow, thereby preventing the pulverizer from being overloaded with coal.

Still another object of my invention is to provide such an apparatus and method wherein the Weight rate of coal and air to the pulverizer is changed in exact accordance with changes in demand.

A further object of my invention is to provide a control wherein the primary air-coal ratio may be easily adjusted to meet the conditions peculiar to an installation but once properly adjustedwill thereafter accurately and precisely maintain the desired primary air-coal ratio throughout the range of operation of the system.

Referring to the drawing, there is shown a rotary kiln 10 which is supplied with pulverized coal entrained in primary air as a carrier from a pulverizer 12. The kiln 10 is provided with a burner 14 which is connected to the pulverizer 12 by a burner pipe 16.

The kiln 10 is provided with the usual hood 18 having a bottom opening 20 for the discharge of the product treated in the kiln ll Secondary air for combustion purposes is supplied the kiln upwardly through the material outlet 2i), the secondary air being preheated in cooling the product discharged from the kiln 10. High temperature furnace gas is' withdrawn through an opening 22 in the upper portion of the hood 18 for tempering the air injected into the pulverizer,

The pulverizer 12 is driven by a constant speed motor 13 and is supplied with raw coal from an overhead bin 24 through a pipe 26 which extends to feeder 28 driven by a variable speed motor 30. The rate of raw coal delivery to the pulverizer 12 is varied by varying the speed of operation of the motor 30.

High temperature furnace gas With-drawn from the hood opening 22 and air from a pipe together forming the primary air are drawn through a conduit 32 by a blower 34 discharging into the pulverizer 12. The conduit 32 is provided with a damper 36 operated by a pneumatic drive 38 to control the rate of flow of primary air to the pul verizer 12. The pipe 40 is similarly provided with a flow control damper 42 operated by a pneumatic drive 44. The damper 42 provides a means for adjusting the ratio of air to high temperature furnace gas to maintain the temperature of the coal-air mixture leaving the pulverizer 12 at a desired value. As will be apparent the damper 42 may, if preferable, be located in the duct carrying the high temperature furnace gas;

As will be apparent to those skilled in the art the invention is not limited to use with any particular type of pulverizer. The pulverizer 12 as illustrated is of the rotating ring and ball type wherein grinding balls rotate in a circular race formed between a lower rotating ring and an upper stationary ring. The raw coal delivered to the grinding zone formed by the grinding elements is at least partly reduced in size and discharged outwardly of the grinding elements into a rising stream of air. The air entering the pulverizer from the blower 34 passes upwardly beyond the grinding rings and lifts the finer particles of coal into a classifying zone in the upper portion of the pulverizer. The classifier rejects the coarser coal particles entrained in the carrier air stream, such particles returning to the grinding zone for further pulverizing. The finished product, pulverized coal, entrained in the carrier air passes through an outace-3,525

let in the upper portion of the pulverizer into the burner pipe 16 and is discharged from the burner 14.

A solids flow meter 54 of the type disclosed in my copending. U.S. patent application Serial No. 854,916, filed November 23, 1959, is associated with the pipe 26 and is efiective to produce rotation of a flexible shaft 56 at a speed proportional to the flow rate of the raw coal to the pulverizer 12. More particularly, the device 54 comprises a rotatable helical vane 58 Within the pipe 26 and connected by an elongated shaft 6% to the flexible shaft 56 by means of parts within the device 54. For a complete description of the structure and operation of this solids flow meter reference is made to my aforementioned copending application.

A transmitting device 62 which is disclosed and claimed in my copending U.S. patent application Serial No. 17,307, filed March 24, 1960, is effective to transduce the rotational velocity of the shaft 56 into a proportional pneumatic signal pressure in tube 64. The pneumatic pressure in tube 64 will thus be directly proportional to and havea linear relationship with the flow rate of the raw coal through pipe 26.

A second transmitting device 66 is responsive to the differential pressure across an orifice 68 in the primary air duct 32 to establish a pneumatic pressure signal in a tube 70 proportional to the flow rate of primary air supplied to the pulverizer 12. The transmitter 66 may take various forms as Well known to those skilled in the art, and therefore, further description is deemed unnecessary.

The pressure signal in tube 64 representative of coal flow is supplied to the B chamber of a pneumatic relay 72.11aving proportional plus reset action. The relay 72 may be of the type disclosed in U.S. Patent No. 2,805,678 to Michael Panich and as described therein may be used to generate a pneumatic pressure at outlet connection D changing in proportion to changes in the differential existing between the A and B inlet connections and the time integral of the differential.

The pneumatic signal at output connection D is transmitted through tube 74 to a selector station 76 which maybe of the type disclosed in U.S. PatentNo. 2,729,222 to Paul S. Dickey et al. and thence to a controller 82 for motor 30. The controller 82 establishes a direct current output proportional to the magnitude of the pneumatic signal from selector station 82 to produce a proportional speed of the motor 30. Thus, the flow rate of coal to the pulverizer 12 is measured by device 54 and the motor 3th is controlled to maintain the flow rate constant at a value determined by the loading pressure at inlet connection A of relay 72.

The pressure signal in tube 70 representative of air flow is supplied to the A chamber of a proportional action calibrating relay 86 which is also of the type disclosed in U.S. Patent No. 2,805,678. The relay 36 adjusted to produce a predetermined relationship between the pressure signal in tube in and the relay output pressure in a tube 88, the relationship being variable through internal proportional band adjustments disclosed and described in the aforementioned patent.

The output pressure in tube 88 is applied to the A chamber of relay 72 to establish the set point for the coal flow control described above. With this. arrangement the set point for the coal flow control is adjusted in ac cordance with the primary air flowand at any value of matic selector station 94 similar to the selector station ,76, and transmitted to the drive 33 to position the damper 36 in accordance with the pressure signal in tube 92. A manually adjustable set point signal for the air flow control may be established by a manual loader 91 and applied by tube 96 to the A chamber of relay 9%. Thus the action of relay 9% Will be similar to that of relay 72, the output pressure signal in tube 92 varying as required to maintain the air flow to the pulverizer proportional to the loading pressure established by device 91.

It will be apparent to those skilled in the art that the set point pressure signal applied to the A chamber of clay may be derivedfrom an associated combustion control or other system (not shown) instead of being manually derived; and further that the loading pressure established by the device 91 may be transmitted through a tube 93 to other pulverizers. As for each loading pressure established by the .device 91 there is a definite metered rate of air flow and coal flow all pulvcrizers will operate at precisely the same load.

Referring now to the control for damper a2, a suitable pneumatic transmitter ill-ll is responsive to the temperature of the primary air-fuel mixture supplied burner 14 through pipe 16 as sensed by a thermocouple or other suitable sensing element 162 to establish a proportional pneumatic pressure signal in tube 164. Similarly, a transmitter 106 is responsive to temperature'of the primary air in conduit 42 as sensed by element 103 to establish a proportional pneumatic pressure in tube 110. Thus, the pressure signals established by transmitters and 166 are representative of the pulverizer outlet and inlet temperatures respectively.

The pressure signal representative of inlet temperature in conduit 32 is supplied to the B chamber of a proportional plus reset pneumatic relay of the type disclosed in U.S. Patent No. 2,808,678 to establish an output signal in tube 112 having proportional plus reset characteristics. The output pressure signal in tube 112 is taken through a manual automatic selector station 114 and applied to the drive 44 to effect positioning thereof.

The outlet temperature signal in tube 104 is applied to the B chamber of a proportional plus reset pneumatic relay 116 producing a signal in tube 126 which is applied to the A chamber of relay 165. A manually adjustable set point signal is established in selector station 114 and applied to the A chamber of relay 116 by tube 122.

The drive 44 is thus positioned from the index of mill inlet temperature as established by the loading pressure at the A connection of relay 1% to adjust the damper 42. The mill inlet temperature index is derived from relay 116 and adjusted as required to maintain the mill outlet temperature at the predetermined value established by the manually adjustable set point in selector station Several factors determine the optimum primary aircoal ratio such as type of burner, type of coal, flame pattern desired, size and configuration of the pipe used to transport the primary air-coal mixture from the pulverizer to the burner, characteristics of the pulverizer and the like. As these factors and their relative efi'ect on operation are ditferent for each installation, it is necessary to especially establish the primary air-fuel ratio. Generally, the primary air-fuel ratio is a variable quantity within the fuel burning equipment operating range. Most pul'verizers are designed to operate at rated load with approximately 15 to 25 percent of the total air used for combustion. This ratio increases with a decrease in pulverizer output.

The pulverizer control I have described is readily adjusted to meet the foregoing requirements. Thus at minimum firing rate and with the desired primary air-fuel ratio established the calibrating relay 86 is adjusted to the null or set point. With the set point so established, the mill output is then increased to approximately maximum and the proportional band of relay 86 then adjusted to establish the proper primary air-fuel ratio. This completely determines the calibration of the'controlfrom minimum to maximum load.

As heretofore described the output of the mill is established by the loading pressure in tube 96. For each and every loading pressure in tube 96 a definite primary air flow will be maintained as my control incorporates a constant air flow control the set pointtof which is adjusted by varying the loading pressure intube 96. Further, there is a definite coal flow maintained for each and every rate of primary air flow so established as the loading pressure in tube 70, representative of primary air flow, after modification as required in calibrating relay 86 establishes the set point of the constant coal flow control.

Pulverizers are subject to plugging it overloaded with coal, that is if coal is fed to the pulverizer in amounts greater than can be handled by the existing rate of primary air flow. Such excessive feeding of coal to the pulverizer is impossible with the control system herein described as the rate. or" coal fiow is established by the rate of air flow actually existing. Thus it may happen that the demand for pulverizer output as established by the loading pressure in tube 96 exceeds the available primary air flow as determined by the capacity of the primary air fan 34-. Such excessive demand will not result in coal being fed to the pulverizer at an excessive rate, even though available, as the loading pressure in tube 96 does not act directly to establish the rate of coal flow, but only to establish the rate of primary air fiow and the coal flow. is then matched to the actual rate of primary air flow.

Accurate temperature control of the pulverizer outlet temperature is maintained by damper 42 which is controlled from inlet and outlet temperature. The damper :2 is positioned from the primary index of pulverizer inlet temperature readjusted in accordance with pulverizer outlet temperature to efiect a constant outlet temperature condition. The control anticipates the efiect changes in the inlet temperature will have on the outlet temperature and immediately corrects for such changes to nullify their effect on outlet temperature.

While only one embodiment of the invention has been herein shown and described, it will be apparent to those skilled in the art that many changes may be made in the construction and arrangement of parts without departing from the scope of the invention as defined in the appended claims.

What I claim as new and desire to obtain by Letters Patent of the United States is:

1. In a control system for an air swept pulverizer having separate solid material and air supplies, the combination comprising, means for varying the flow rate of solids to the pulverizer,.means responsive solely to variations in the actual flow rate of solids entering the pulverizer to control said solids varying means to maintain a predetermined solids flow rate, and means responsive to variations in the actual flow rate of air to the pulverizer for proportionally modifying said predetermined solids flow rate to the pulverizer to maintain a predetermined relationship between the amount of air and solid material entering the pulverizer.

2. In a control system for an air swept pulverizer having separate solid material and air supplies, the combination comprising, means for varying the flow rate of solids to the pulverizer, means for varying the flow rate of air to the pulverizer, means responsive to variations in the actual fiow rate of solids entering the pulverizer to control said solids varying means to maintain a predetermined flow rate of solids, means responsive to the actual flow rate of air to the pulverizer for controlling said air flow varying means, and means including the last said means for producing a calibrated variation in said predetermined solids flow rate in accordance with variations in the actual flow rate of air to the pulverizer.

3. In a control system for an air swept pulverizer having separatesolid material and air supplies, the combination comprising, means for varying the flow rate of solids to the pulverizer, means for varying the flow rate of air to the pulverizer, means responsive to variations in the actual flow rate of solids entering the pulverizer to control said solids varying means, means responsive to variations in the actual flow rate of air to the pulverizer for controlling said air flow varying means, and means for modifying control of said solids varying means from said solid flow responsive means in accordance with variations in the actual flow rate of air to the pulverizer. I

4. In a control system for an air swept coal pulverizer having separate coal and air supplies, means for varying the fiow rate of coal to the pulverizer, means for establishing a first signal representative solely of the actual flow rate of coal entering the pulverizer, means for establishing a set point signal, means for positioning said coal flow varying means in accordance with the difierence between said first and said. set point signals, means responsive to the actual flow rate of air to the pulverizer to establish a second signal, and means for varying said set point signal in accordance with variations in said second signal.

5. In a control system for an air swept coal pulverizer having separate coal and air supplies, the combination comprising, means for varying the fiow rate of coal to the pulverizer, means for establishing a signal representative of the actual flow rate of coal entering the pulverizer, means for establishing a first set point signal, means for controlling said coal flow varying means in accordance with the difference between said actual coal flow and said first set point signals, means for varying the flow rate of air to the pulverizer, means responsive to the actual flow rate of air to the pulverizer to establish a signal representative of the actual air flow to the pulverizer, means for establishing a second set point signal, means for controlling said air flow varying means in accordance with the difference between said actual air flow and said second set point signals, and means for proportionally varying said first set point signal in accordance with variations in said actual air flow signal to maintain a predetermined relationship between the air flow rate and coal flow rate.

6. The method of controlling the supply of solid material and air to an air swept pulverizer which includes the steps of, controlling the rate of fiow of solid material to the pulverizer solely in accordance with variations in actual flow rate of solids entering the pulverizer to maintain a predetermined flow rate of solids material, and varying the predetermined solids flow rate in accordance with the actual flow rate of air to the pulverizer to maintain a predetermined relationship between the flow rate of air and solid material to the pulverizer.

7. The method of controlling the supply of solid material and air to an air swept pulverizer which includes the steps of, controlling the rate of fiow of solid material to the pulverizer in accordance with variations in the actual flow rate of solid material entering the pulverizer to maintain a predetermined flow rate of solid material, controlling the rate of flow of air to the pulverizer in accordance with the actual flow rate of air to maintain a predetermined flow rate of air, and proportionally var ing the predetermined flow rate of solid material in re sponse to variations in the rate of air flow to maintain a predetermined relationship between the flow rate of air and solid material to the pulverizer.

8. The method of controlling the supply of solid material and air to an air swept pulverizer which includes the steps of, measuring the actual flow rate of solid material entering the pulverizer and establishing a signal representative of the magnitude thereof, comparing the solids flow signal with a solids flow set point signal, controlling the rate of flow of solid material in accordance with the difierence between the solids flow signal and set point signal, measuring the rate of actual air flow to the pulverizer and establishing a signal representative of-the magnitude thereof, comparing'the actual air flow signal with an air flow set point signal, varying the rate of air flow in accordance With the difierence in magnitude of the air flow signal and air flow set point signal, and producing a calibrated variation in the solids flow set point signal in accordance with variations in the actual air flow signal.

UNITED STATES PATENTS Schwartz Aug. 4,

Mosshart Oct. 23,

Dickey Apr. 13,

Dickey Nov. 14,

FOREIGN PATENTS Canada June 6, 

